Known in the art is a method for determining mechanical properties of articles made from ferromagnetic materials (cf. Forster, Zeitschrift fur Metalkunde, Bd 45, H 4, 245, 1954), in which the test article is magnetized through a single exposure to an axially symmetric magnetic field normal to the surface of said article. Mechanical properties of the article being tested are determined by the magnitude of the tangential component of a local magnetic field of remanent magnetization. The method is carried out with the aid of a device wherein as a magnetizing means use is made of a linear permanent magnet.
However, the results obtained through a single magnetization are not stable and therefore cannot be reproduced unless the article being examined is demagnetized before a repeated measurement is made. Besides, said device is highly sensitive to a gap between the permanent magnet and the article, and to a structural anisotropy of the material from which the article is fabricated.
There is also known a method for determining mechanical properties of articles made from ferromagnetic materials (cf. Melgui M. A. "Magnitny kontrol mehanitcheskih svoistv staley", Nauka i tekhnika, 1980, p. 140), wherein the test article is magnetized by repeatedly exposing it to a pulsed axially symmetric magnetic field having a constant amplitude. The symmetry axis of said magnetic field is maintained normal to the surface of the article. Then the normal component gradient of the magnetic field of remanent magnetisation is determined along the symmetry axis of said magnetic field with the aid of a ferroprobe-gradiometer. The method rules out the influence of the magnetic history in the case of thin articles and provides for stable results in testing such articles.
However, indefinitness of the depth of magnetization in the case of articles having a greater thickness and the influence of the magnetic history of such articles affect the stability and precision of the testing results. For the above reasons this method cannot be used for testing such articles as, for instance, rolled plates thicker than 4 mm.
A device for carrying out the above method (cf. "Defectoskopiya", 1979, No. 3, p. 29) comprises a pulse shaper adapted to form pulses of an axially symmetric magnetic field, a circuit for measuring a gradient of remanent magnetization, and an indicator, all the above elements connected in series. The pulse shaper is contructed in the form of a magnetizing device (solenoid) connected to a conventional current pulse generator.
Magnetizing the test article is effected by the pulses of the solenoid magnetic field. The solenoid is installed so that its end face is adjacent to the surface of the test article. A signal from a transducer adapted to convert a gradient of the normal component of remanent magnetization to electric signals is applied to the measuring circuit from which it is transmitted to the indicator. The readings of the indicator are used for determining mechanical properties of the article being examined.
The above apparatus provides for reliable test results in testing articles having a thickness less than 4 mm. However, in controlling the quality of a product after a thermal treatment and determining mechanical properties of articles having a greater thickness, the recurrence and, hence the reliability and precision provided by such quality control are lower.